Reactance tube circuit



1945. F. G. MARBLE Q A 2,382,436

REACTANCE TUBE CIRCUIT Filed Aug. 18, 1943 CIRCU/TOF m5 PRIOR ARTCONTROL 5/ VOLTAGE 9 k CONTROL VOLTAGE AMI? 2 RECT. [g l-- INVEN T 0/? EG. MARBLE BY ATTORNEY Patented Aug. 14, 1945 REACTAN CE TUBE CIRCUITFrank G. Marble, West Orange, N. 3., assignor to Bell TelephoneLaboratories,

Incorporated,

New vYork, N. Y., a corporation of N ewYork Application August 18, 1943,Serial No. 499,158

4 Claims.

This invention relates to an improvement in reactance tube circuits,particularly with reference to the use of such circuits as reactancesvarying with a direct current control voltage.

The object of the present invention is to provide a reactance tubecircuit constituting a capacity capable of being varied in linearproportion to a unidirectional voltage applied to the input circuit ofthe rectance tube.

Numerous circuits known to the prior art provide electronic reactancesvarying with a control voltage and most commonly applied in theautomatic frequency control of a radio receiving circuit localoscillator wherein the control voltage is derived by rectification of aportion of the intermediate frequency wave derived from the signal wavebeing received. It suflices for such use that the circuit operates toestablish a definite intermediate frequency and no requirement ex-- iststhat such frequency be approached linearly with variation in thecontrolling voltage. In other words, the trimming capacity must reach adesired value without specification of its approach thereto.

A stricter requirement must be fulfilled if the reactance tube circuitis to cause an effective capacity to vary linearly with a voltagerepresenting a physical condition, a manner of capacity variationdesired, for example in the vibra tion analyzer disclosed and claimed inthe copending application Serial No. 483,129, filed April 15, 1943, byW. J. Brown. The analyzing'circuit described by Brown includes anoscillator of which the tuning is varied by a voltage responsive devicewhich varies the capacity in the frequency determinin circuit of theoscillator linearly with a voltage proportional to the rotational speedof an airplane engine. The required variation in frequency is from92,000 to 92,500 cycles per second, and over such a narrow range issensibly linear with variation of the capacity in the tank circuit ofthe oscillator. It is clearly a saving in equipment to substitute forthe voltage responsive device and the condenser thereby controlled areactance tube 'constituting a capacity reactance directly controlled bythe voltage proportional to the engine speed.

Therefore, it is another object of the invention to simply the circuitof a vibration analyzer.

The invention will be fully understood from the following descriptionread with reference to the accompanying drawing in which:

Fig. 1 shows a circuit of the prior art in which a reactance tubecircuit constitutes a variable capacity; and

Fig. 2 shows a circuit generally similar to that of Fig. 1 but includingthe improvement of the present invention. In both figures like numeralsidentify like elements. Alternating voltages and currents arerepresented by small letters, unidirectional voltages by capitals.

In Fig. 1 tube 1 is preferably a pentode such as the 6.407, cathode 2and control grid 3 are shunted by inductance Ll and condenser l5 whilegrid 3 and anode 6 are connected by the feedback path comprisingresistor R1 in series with blocking condenser C1. Cathode 2 is heated bythe usual means not shown and is grounded. Suppressor grid 5 is joinedto cathode 2 while screen grid 4 is connected to a suitable potential onbattery I from which the supply of current to anode 6 is throughinductance 1.12.. Grid 3 is negatively biased with respect to cathode 2by means of battery 12. Inductance L2 and con denser C2 plus thecapacity represented by tube l constitute the frequency determiningcircuit of oscillator H] which generates a voltage of frequency saykilocycles per second. The space current and so the mutual conductanceof tube I is controlled by a direct current voltage across terminals 6and 9 in series between battery I 2 and grid 3, a high resistance Bbeing included in series between terminal 8 and grid 3. Con denser I5 isa blocking condenser of large capacity. Resistances R1 and R2 are eachof the order of 1 megohm. The capacity of condenser C1 is suitably ofthe order of 0.1 microfarad. The voltage on screen grid 4 and that ofbattery I are respectively of the order of 100 and 300 volts when tube lis a 6AC7. For the illustrative frequency of 100 kilocycles per secondLl. and C2 plus the capacity represented by tube I may be 1 millihenryand 2530 micromicrofarads, respectively.

Circuits such as that of Fi 1 are well known and reference may be madeto the article by C. Travis entitled Automatic frequency control in thePro-c. I. R. E. vol. 23, page 1125, October, 1935. It is necessary thatthe feedback resistance R1 and the internal resistance of tube I be verylarge in comparsion with the resonant impedance of circuit L2 and C2 andthat resistances R1 and R2 be both very large compared with theimpedance of L1 at the resonant frequency. The tube and its associatedcircuit constitutes an ef-- fective condenser, shunting the tankcircuit, of capacity where Gm is the mutual conductance of tube 1.

A control voltage impressed across terminals 8 and 9 serves to determinethe conductance Gm which accordingly varies with the variation of thecontrol voltage itself.

In a practical case, the capacity of condenser C2 may be 2280micromicrofarads and inductance L2 ma be 1 millil'lenry. If R1 equalslmegohm and L1 50 millihenries, Gm may readily be made 5000 micromhos sothat the effective capacity shunting tank circuit L2 C2 becomes .250micromicrofarads so that the total capacity of the tank circuitcorresponds to a resonant frequency of 100 kilocycles per second. Withthe polarity of the control voltage shown in Fig. 1, an increaseinmagnitude of this voltage reduces the mutual conductance, and so theeffective capacity shunting the tank circuit, thereby increasing thefrequency of oscillator I0. If for inductance L1 a condenser of suitablysmall capacity is substituted the circuit of tube I becomes an effectiveinductance varying inversely with Gm, an arrangement which in some casesis desirable. For many purposes, an example being the vibration analyzerdisclosed in the Brown application referred to, it is preferred toemploy the circuit of Fig. 1 and vary the oscillator frequency byvarying the total capacity of the tank circuit in accordance with acontrol voltage.

In Fig. 1, condenser C2 is shunted by the effective capacity L G... R1

An alternating current z' flows in the anode circuit of tube I dueprincipally to the alternating voltage 9g which appears between grid 3and ground. This voltage 6g on grid 3 is the alternating component wherew is 21r times the instantaneous frequency of en the voltage acrossoscillator Ill. The ratio ip/eg, or Gm, depends on the bias voltage ofgrid 3 and this is the voltage of battery I2 together with the externalcontrol voltage E1 between terminals 8 and 9. A deliberate variation ofE1, therefore results in a variation in G111, entailing a variation inthe frequency generated by oscillator I resulting from the capacityvariation of the reactance circuit, a capacity variation which would belinear with that of E1, were the characteristic of mutual conductancevs. grid voltage a straight line.

The advantage for the above purpose of the circuit of Fig. l i itsgreater compactness as compared with mechanical devices. On the otherhand, since the curve of mutual conductance vs. grid voltage is S-shapedthere is the disadvantage that the effective capacity of a reactancetube is not a linear function of the control voltage except over a verynarrow range. This disadvantage is largely removed by the presentinvention and the manner of its removal will appear from the descriptionof the circuit of Fig. 2.

Referring now to Fig. 2 a small resistance I I is inserted betweenground and cathode 2. From a suitable point on resistor II a connectionI3 is taken to amplifier I4, the rectified output of which is introducedin series between ground and terminal 9 and is so poled as to be inopposition to the control voltage applied between terminals 8 and 9.Resistor I I is traversed by a complex current composed of theunidirectional space current of tube I together with an alternatingcurrent due to the fraction of the oscillator voltage from tank circuitL2 C2 introduced across L1. Under the assumptions made above themagnitude of this alternating component depends on the mutualconductance of tube I as does the effective capacity presented by thecircuit of Fig. 1. As the control voltage increases in magnitude, grid 3goes more negative, the mutual conductance decreases and the alternatingcomponent of the current in resistor I I decreases. The correspondingalternating voltage applied b connection I3 to amplifier rectifier I4emerges as a unidirectional voltage introduced in opposition to thecontrol voltage.

Resistor II is chosen to be a sufficiently low resistance, say 25 ohmsto avoid the introduction of negative feedback other than that it isdesired to introduce by the circuit connection including amplifierrectifier I4. Only the alternating component of the current traversingresistor II is useful for the purpose of the invention, for which reasonblocking condenser I6, of large capacity, is included in series betweentap I3 and the voltage divider consisting of resistor IT in eries withcondenser I8 to ground. Across condenser I8 is connected the input toamplifier rectifier I 4, the input resistance I9 of which is representedin dotted line. This input resistance is of the order of 1 megohm, largecompared to the reactance of condenser I8 and it is convenient to makethe resistance of resistor I 1 25,000 ohms. The capacity C18 ofcondenser I8 is suitably chosen to be equal to where R17 is theresistance in series with condensers I6 and I8 and w is 27r times thefrequency one octave lower than the lowest operating frequency ofoscillator I0. It may be shown that under these conditions, forvariations in frequency of oscillator l0 above the lowest operatingfrequency, the ratio e1/eg of the voltage e1 across condenser I8 to thegrid voltage 6g involves the operating frequency inversely as does thevoltage ratio eg/eo, wherefore the ratio e1/eo is independent of theoscillator frequency over the range assumed.

It will be recognized that the disposition of apparatus shown in Fig. 2makes use of a special kind of inverse feedback. In the usual feedbackcircuit a portion of the output voltage of an amplifier is fed back inreverse phase to the input circuit thereof and thereby deformities inthe output energy are corrected so that the output energy becomes a morefaithful enlargement of the input energy. In the circuit of the presentinvention the deformation to be corrected arises from the characteristicof the reactance tube and is represented in the alternating currenttraversing resistor II. Since the variation of a unidirectional controlvoltage is to be preformed to compensate in advance for the tubecharacteristic, rectification is required of the compensatingalternating voltage and the required reversal of phase is obtained bypoling the rectified voltage in series opposition to the control voltagewhich it is desired to modify. The magnitude of the compensating voltagemay be altered either by altering the point of connection I3 on resistorII or by a gain control in amplifier rectifier I4, whichever is moreconvenient. In practice it is a simple matter to determine the requiredcompensation for a tube of given type by plotting a family of curves ofcontrol voltage vs. anode current for various adjustments of thecompensating voltages. Once this determination is made, the anodecurrent and so the mutual conductance of the tube over a wide range varylinearly with respect to the control voltage (E1 in Figs. 1 and 2)although not so with respect to the total grid to cathode voltage(El-E2, Fig. 2). The efiective capacity of the reactance in the circuitof Fig. 2 likewise varies over the same range linearly with the controlvoltage thereby attainin the object of the invention.

What is claimed is:

1. In an electronic reactance circuit including a thermionic vacuum tubeprovided at least with a control grid, a cathode and an anode andcontrolled by a unidirectional voltage applied in series between saidcontrol grid and ground, means for varying the mutual conductance ofsaid tube linearly with said voltage comprising a source of alternatingvoltage connected between said ancomprising a first resistance in serieswith a capacity between said control grid and said anode, a firstreactive impedance connected between said control grid and ground, asecond resistance in series between said cathode and ground, an outputcircuit for said tube including a second reactive impedance connectedbetween said anode and ground, an input circuit for said tube shuntingsaid first reactive impedance and including portion of the voltageacross said second resistance and means for introducing in series withsaid third resistance said external voltage in selies with and inopposition to said rectified voltage.

.3. An electronic reactance circuit adapted to be controlled by aunidirectional voltage, comprising a thermionic vacuum tube provided atleast with a control grid, a cathode and an anode, power supply for saidtube, an impedance coupling said control grid and said anode, a sourceof alternating voltage connected between said anode and ground, aresistance in series between said cathode and ground, means for rec'-tifying the voltage across said resistance, an input circuit for saidtube including a second impedance and means for introducing in serieswith said second impedance said external voltage and said rectifiedvoltage in mutual opposition.

4. Means for controlling the tuning of the frequency-determining circuitof an oscillator linearly with the magnitude of a unidirectional controlvoltage comprising a thermionic vacuum tube provided at least with acontrol grid, a cathode and anode, power supply for said tube, an outputcircuit for said tube, means for connecting said frequency-determiningcircuit in parallel with said output circuit, coupling including a firstresistance in series with a capacity between said anode and said controlgrid, an inductance connected between said control grid and ground, asecond resistance in series between said cathode and ground, an inputcircuit for said tube including a third resistance, means for rectifyinga desired portion of the voltage across said second resistance and meansfor connecting said control voltage and said rectified voltage in mutualopposition in series between said third resistance and ground.

FRANK G. MARBLE.

